There are several sections of this project involving different aspects of imaging. 1. MR sequences: We are currently working in the Laboratory of Diagnostic Radiology Research evaluating multiple MR sequences on children with brain tumors, longitudinally, to assess outcome to tehrapy and prognostic factors. These imaging modalities include spectroscopy (single voxel, CSI and multislice multivoxel), perfusion studies (DEMRI, DSC-MRI) and diffusion tenor imaging. 2. Results from NMRS are being compared to the "gold standard" in adult neuro-oncology, i.e. FDG-PET, to determine the correlation between results of each modality. 3. New nuclear medicine studies involving alternate radioisotopes are being evaluated. We have opened a study involving pediatric patients with high grade gliomas or diffuse brainstem tumors who will be evaluated by FLT, FDG and NMRS.The objectives of this study specifically are: Determine the feasibility of 18F-FLT PET imaging in pediatric patients with malignant gliomas Determine the ability of 18F-FLT PET imaging to detect treatment changes in pediatric patients with malignant gliomas undergoing radiation therapy Determine the safety and toxicity profile of PET imaging using 18F-FLT in pediatric patients with malignant gliomas To correlate changes in 18F-FLT PET before and after radiation with outcome (12 month PFS) To compare the performance of 18F-FLT-PET to that of MR perfusion, proton magnetic resonance spectroscopy (1H-MRSI) and 18F-FDG PET in prediction of tumor response, time to progression and overall survival time.